1. Field of the Invention
The present invention relates to a developing device using a two-component developer containing toner and carrier and an image forming apparatus provided with the same.
2. Description of the Related Art
There is known a developing device using a two-component developer containing nonmagnetic toner and magnetic carrier. This developing device generally includes a developer conveyance path for conveying the developer, a developing roller for supplying toner from the developer conveyance path to a predetermined image bearing member (e.g. photoconductive drum) to form a toner image on the image bearing member, and a screw feeder extending in the developer conveyance path for conveying the developer while agitating it. As the developer is agitated by the screw feeder, the non-magnetic toner is charged by the magnetic carrier.
A toner supply port for supplying new toner into the developer conveyance path is provided at a position above the developer conveyance path. The toner supplied into the developer conveyance path is conveyed in the developer conveyance path by the screw feeder. When the new toner is supplied into the developer conveyance path, a specified amount of toner falls in bulk to a position below the toner supply port in the developer conveyance path. To satisfactorily charge the toner and make the toner density of the developer uniform, the toner supplied in bulk needs to be quickly dispersed into the developer.
There is known a developing device employing a technology for quickly dispersing toner. In this developing device, a shaft portion of a screw feeder includes a plurality of needle members projecting in radial directions of the shaft portion and a blade portion of the screw feeder includes a plurality of needle members projecting in an axial direction of the shaft portion. The plurality of needle members are arranged below a toner supply port. Thus, even if a specified amount of new toner is supplied in bulk, the plurality of needle members quickly disperse the toner into a developer as the screw feeder is rotated.
However, in the above developing device, the toner is likely to be accumulated in clearances between the inner surface of the developer conveyance path and the screw feeder and the plurality of needle members although the newly supplied toner is dispersed by the plurality of needle members. It is difficult to disperse the toner accumulated in the clearances into the developer by the screw feeder and the needle members. If the accumulated toner agglomerates and is conveyed into the developer conveyance path at a certain moment, it becomes difficult to satisfactorily charge the toner, wherefore so-called fogging occurs due to toner charging failure. As a result, it is difficult to form a good toner image.